In the manufacture of semiconductor chips and semiconductor chip packaging substrates, semiconducting and dielectric materials are employed. In a semiconductor chip, conductive circuit patterns are provided on the semiconductor materials and on dielectric materials disposed on the semiconducting material. In semiconductor chip packaging substrates, conductive circuit patterns are provided on dielectric materials which are typically glasses, ceramics, polymeric materials and combinations thereof. In order to electrolessly plate metal on substrates formed of these materials, it must be seeded or catalyzed prior to the deposition of the electroless metal thereon.
Among the more widely employed procedures or catalyzing a substrate is the use of a stannous chloride sensitizing solution and a palladium chloride activator to form a layer of metallic palladium particles thereon. For instance, one method of catalyzing a dielectric substrate is exemplified by U.S. Pat. No. 3,011,920 which includes sensitizing a substrate the first treating it with a solution of colloidal metal, then accelerating the catalyst by treatment with a reagent to remove unreactive regions from the colloids on the sensitized dielectric substrate, and electrolessly depositing a metal coating on the sensitized substrate, for example, with copper from a solution of copper salt and a reducing agent.
Unlike the prior art, the present invention does not use a stannous chloride sensitizing solution. The present invention first treats the surface with a pretreatment which disposes at the surface of the substrate a species, preferably an anionic species, capable of transferring electrons. The surface of the substrate is then contacted with the solution of another species preferably a metal cation or a metal cation-containing compound. The species disposed on the surface is capable of transferring an electron to the species in solution. When the electron transfers, metal of the compound in solution is disposed on the surface of the substrate in reduced form, preferably in the zero valence state of the atom corresponding to the species in solution. Preferably, the species in solution is a metal cation or a metal complex which in the zero valence state is catalytic to the electroless deposition of metal thereon.
U.S. Pat. No. 4,261,800 describes a method For electroless deposition of metal onto a nonconducting substrate, such as polyimide. A reducing agent solution of hydrazine or substituted hydrazine is deposited onto a surface of the substrate. The reducing agent in the solution is either absorbed onto the surface or remains on the surface. The surface is selectively exposed to ultraviolet radiation causing decomposition of the hydrazine. The surface is contacted with a solution containing an activating metal species. The irradiated parts of the surface are incapable of reducing the activating metal species. In the nonirradiated part of the surface, the activating metal is deposited on the surface which is useful as seed For subsequent electroless deposition. The activating metal species can be platinum, or palladium.
U.S. Pat. No. 4,112,139 describes the metallization of polyimide surfaces by exposing the surface to a substituted or unsubstituted hydrazine. The treated surface is photolytic, which can be selectively desensitized by exposure to U.V. light. When the treated substrate is placed in a palladium chloride solution, palladium metal is disposed on the unexposed regions and no palladium is disposed on the exposed regions.
Substituted and unsubstituted hydrazines are toxic and highly explosive chemicals which therefore are unsuitable for large scale manufacture of electrical components. In contradistinction, the compounds, sorbed at the surface to be metallized according to the present invention, are not highly toxic and are not highly explosive.
It is an object of this invention to provide a reducing agent at the surface of a substrate which is free of hydrazines.
U.S. Pat. No. 4,775,449 describes improving the adhesion of metal applied on a polyimide surface by treating the surface with an adhesion promoting compound containing a nitrogen-oxygen moiety. It is suggested that the compound acts as a reducing agent and is incorporated into the polyimide either along the diamine-based chain portion or into the dianhydride based portion of the polyimide. After treating the surface with the adhesion-promoting compound, the substrate may be thoroughly rinsed with distilled water or a methanol solution and then activated For plating by methods well-known in the art.
In contradistinction, the compounds of the present invention which are applied to a substrate do not chemically alter the substrate surface. According to the present invention, a species, preferably an anionic species, is provided at the surface of the substrate. This species does not chemically alter the substrate, but is capable of transferring an electron to a species in solution resulting in the disposition on the surface of the substrate a reduced form of the species in solution.
It is another object of the present invention, to provide a reducing agent at the surface of the substrate which does not chemically alter the substrate.
U.S. Pat. No. 4,782,007 describes a process for making printed circuit boards using aqueous alkaline strippable resist. The solution to strip the resist preferably includes, a reducing agent such as hydrazine, alkali, hypophosphite, sodium borohydride, dimethyl amino borane, or other like reducing agents, which appear to have the affect of improving the subsequent electroless deposit on the catalyzed substrate either by mininizing removal of the catalyst during stripping or stabilizing catalysis or exposing additional catalytic surface. Prior to application of the stripping solution containing the reducing agent, the resist-patterned substrate is treated with the solution which deposits a species catalytic to electoless deposition.
U.S. Pat. No. 3,488,166 to Kovac et al. describes a method for activating a formaldehyde containing plastic substrate for subsequent metallization by polymerizing on the surface of the plastic substrate a coating of a formaldehyde compound selected from the group consisting of urea formaldehyde plastic, phenolic formaldehyde plastic, melamine formaldehyde plastic and arylsulfonamade-formaldehyde resins. The formaldehyde surface coating is a reducing agent for a metal salt. The formaldehyde coating is sensitized by being contacting with a reducible catalytic metal salt solution for a time to chemically bond the metal constituent of the salt to the coating. The Kovac et al. method requires that the formaldehyde coating be deposited on a formaldehyde containing substrate. In contradistinction, according to the present invention, there is no such requirement.
U.S. Pat. No. 4,448,804 to Amelio et al. describes a method for metallizing a nonconductive surface wherein the surface is treated with a quaternary phosphonium or quaternary ammonium group which provides a positive charge to the surface. Thereafter, the surface is treated with a seeding solution containing tin chloride and palladium chloride which forms a solution of negatively charged colloidal particles containing Pd and Sn surrounded by chloride ions. The negatively charged particles are electrostatically attracted to the positively charged surface. The substrate is treated with HCl to remove Sn leaving Pd on the surface which is catalytic to subsequent electroless plating. In contradistinction, the present invention does not require a charged palladium/tin colloid solution.
U.S. Pat. No. 4,752,529 described depositing within films, which do not have to be electroactive, metal interlayers by reacting metal ions, for example Pd ions, diffusing from a solution in contact with one side of the film with a solution of reducing agent diffusing from a solution in contact with the opposite side of the film. The reducing agent solution can be NaBH.sub.4 in a polar aprotic solvent. This requires that the reducing agent be able to diffuse through the film. In contradistinction, the method of the present invention does not require the reducing agent to be able to diffuse through the substrate.
U.S. Pat. No. 4,668,354 describes disposing a rectal interlayer within a polymeric film. One side of the film is on a cathode, the other side of the film is contacted with a metal cation containing solution. An electroactive transfer compound within the film picks up an electron at the cathode, diffuses into the film where it transfers electrons to metal cations diffusing from the other side of the film to deposit zero valence metal atom in the film. This requires that the reducing agent be able to diffuse through the film. In contradistinction, the method of the present invention does not require the reducing agent to be able to diffuse through the substrate.
According to one aspect of the present invention, a substrate is immersed in an electrolytic cell within which is generated a reducing agent which is sorbed at the substrate surface. The substrate is subsequently contacted with a solution containing a species preferably a metal cation, which is reduced at the substrate surface, most preferably to the zero valence state of the cation.
It is another objective of the present invention to condition the surface of a substrate for subsequent electroless metal deposition by providing at the surface a chemical reducing agent which reduces a species in solution to dispose on the surface a form of the species in solution in a reduced valence state.
It is another more particular object of the present invention to electrochemically generate the reducing agents.
It is another more particular object of the present invention to generate a reducing agent by reacting a species with an alkali metal to produce an anionic form of the species.